Lithospheric-scale scale Three-dimensional Modelling (Application to the EARS and Plateau) Girma Woldetinsae (Geological Survey of Ethiopia)
and H.-J. Götze R. Hackney S. Schmidt (Institut für Geowissenschaften Christian Albrechts Universität zu Kiel, Germany)
Aims Homogenise existing gravity surveys; Compile constraining data and information; Investigate isostasy and isostatic state; Investigate the effect of dynamic topography; Carry out 3-D 3 density modelling; Study the rigidity of the lithosphere.
Regional setting Zagros Orogenic front Nubia Western Rift Image source: SRTM Red Sea Contents WEP Database Eastern Rift Afar MER EEP Arabia 20 N Topography, tectonics, geology Gulf of Aden Somalia MER=Main Ethiopian Rift EARS=(MER, Afar, Eastern Rift systems). Methodology TC=Tanzanian Craton Isostasy 0 Examples from the 3D modelling TC Indian Key results ocean of the 3D modelling Summary WEP=Western Ethiopian Plateau EEP=Eastern Ethiopian Plateau
Topography and tectonic setting Merged topography and bathymetry, data source: GTOPO30, Smith and Sandwell (1997) h = Earthquakes t = Volcanoes
Database
Gravity database Gravity stations: (~ 45000) 1. Geological Survey of Ethiopia (GSE) (black( black) 2. Bureau Gravimetrique Internat. (BGI) (Land: green, Sea: blue) 3. GETECH and Sudan Geological Authority (red) Important data gaps unsurveyed confidential/or or not accessible Estimated Error: 1.5-4.5 x10-5 m/s 2
Bouguer anomaly map Positive Bouguer anomalies: along fracture systems of Gulf of Aden, Indian Ocean and Red Sea axial zone. Relative positive anomalies: Afar, Anza cross rift,, Eastern Sudan. Negative anomalies MER, KR, Western Rift, Plateaus of Ethiopia, Kenya and Saudi Arabia. Seismic experiments Berckhemer et al. 1970 (Ethiopia)) and KRISP in Kenya EAGLE (2001-2004) 2004) X10-5 m/s 2 3-D modelling area
Geometry and density information Topography and geography Geology, tectonics Crustal densities Density measurements in Ethiopia (~ 800) Density information from eastern Sudan Density from chemical composition data Constraints from seismic Afar experiment (1970); KRISP (1990) & EAGLE (2001-2004) 2004) Axial thinning:turkana (KRISP, 1900); NMER (Keller et al. 2004 and EAGLE). Low velocity: (7.4-7.8 km/s) in Afar (Berckhemer,1975). Velocity-density conversions: Sobolev & Babeyko (1994)
Methodology
3-D density modelling: process
Isostasy Why is it necessary to investigate isostasy? For more constraints (e.g. isostatic Moho) For geological correlation Isostatic models Interpretation Study the effect of dynamic topography
Isostatic models Vening-Meinesz (VM) modelled Moho (D= 10 22 Nm, Te= 10 km), Isostatic regional gravity
Isostatic residual field Woldetinsae & Götze (2005)
Dynamic topography Dynamic topography ~600m (Grand) ~400m (Slab) Dynamic topography is generated by flow within the mantle Total topography= (static+dynamic) Crust Mantle Sublithospheric mantle Moho? Regional compensation Local compensation (Airy)
Dynamic topography & isostatic residual field Grand Slab Dynamic topographic models Data source: Lithgow-Bertelloni, pers. comm. Residual fields (x10-5 m/s 2 )
Modelling
IGMAS Howfeatures we define geometry? Geometry input Automatically triangulated geometry Graphical integration of constraining data Interactive modification ASCII output, postscript Calculation of: Gravity, gravity gradients, potential, geoid undulation, remanent & induced magnetic field.
General structure of the model Densities: Mg/m 3
Modelling results and interpretation Measured gravity Modelled gravity
Crustal models N Red Sea (A) Somalia
Crustal models Red Sea N (B) Somalia
Crustal models N Red Sea (C) Somalia
Rift axial
Rift perpendicular
Wide to narrow rift
Wide to narrow rift
Horizontal cross-sections sections
Moho and basement
Moho from 3D model Shallowest Moho in Afar: ~16 km. Deepest Moho in WEP: 48km. Mean Moho: ~30km. The maximum load in WEP: ~8x10 18 kg/m 2 and induces a downward flexure of the Moho from average 35km to about 45km.
Basement topography 12 N Basement topography varies from few 100 m to 7 km. 8 N Deep basement exists in the rift, south west Ethiopia, Afar, Turkana and Eastern Sudan related to sedimentary structures. 4 N Shallow basement corresponds to the Precambrian structures.
Rigidity
Effective elastic thickness/rigidity Coherence/admittance Require large areas Difficult to include internal loads Methods and results are in many cases controversial (Convolution method, e.g. Braitenberg et al. 2002) Possible to include both external and internal loads No need to calculate admittance Higher resolution is possible Input: Moho geometry and total load from a 3D model
Rigidity TGD TGD Summary results MER Rigidity estimates from this work and previous work are different in MER. Turkana...without internal load Te All the models show low rigidity in highly tectonized zones of Afar and Turkana. Precambrian areas have medium to high rigidity....with internal load In Afar, the TGD is marked by change of lithospheric strength in all models.
Summary A new consistent regional gravity database; Bouguer gravity and isostatic residual maps; Isostatic models; A new regional 3-D density model using old and recent constraints; Moho and basement topography maps constrained where possible;
The 3D model offers quantitative estimates of sedimentary thicknesses; The controls on rift architecture are: sediment loading, asthenospheric upwelling (40 km depth and 300 km wide) ) and lower-crustal modifications; Elastic thickness estimates: Te Afar and Turkana: : 5-205 km; Te Plateau (WEP, EEP), MER: 40-60 km; Te Western rift, eastern Sudan basin: 20-30 km; Te Sudan craton: : 50-60 km.
Acknowledgement I thank The Geological Survey of Ethiopia (GSE) for permission to use most of the data sets and information. The research is supported by a grant from KAAD,, Germany. Gysers in Dallol-Afar (Ethiopia) http://www.dankalia.com/index.htm All members at the Institute of Geosciences of CAU and FU Berlin for useful discussions and friendship.
Rigidity TGD= Tendaho-Goba Goba adad discontinuity Without dynamic topography Slab dynamic topography TGD...Without internal load Te MER MER Turkana Turkana...With internal load
Fault systems Latitude Major rivers fault systems (CGMW, 1968) Coast line Longitude
Isostasy
Rigidity
Rigidity estimation Pseudo topography (PpT) Source: http://userpage.fu-berlin.de/~hajo/bratislava/files/isostat/isostat.html